With reference to FIGS. 1 and 1A, multilayer capacitors or chips 10 are commonly used for bypass, coupling, or energy storage applications in electronic circuits. The capacitor 10 includes internal parallel plates 12 and a dielectric body 14, typically made of a ceramic. Alternating parallel plates 12 are connected by respective terminations 16, 18. Each of the end terminations 16, 18 electrically couples to corresponding plates 12 and provides an external electrical connection to the multilayer capacitor 10. As shown, in some applications multiple capacitors 10 may be stacked one on top of the other to create a combined capacitance equal to the sum of the capacitance of the individual capacitors 10. Each of the capacitors 10 in the stack may then be soldered at each end termination 16, 18 to a pair of opposing lead frames 20, 22 by corresponding solder joints 24, 26. The solder joints 24, 26 electrically and mechanically couple the individual capacitors 10 to the lead frames 20, 22 and form a stacked multilayer capacitor assembly 30.
The stacked multilayer capacitor assembly 30 may be subsequently soldered to a circuit board 32, often referred to as a printed circuit board (PCB). As is known, circuit board 32 may include a plurality of other electrical components (not shown), which may be electrically connected using conductive tracks or using other electrical connections (not shown). The circuit board 32 may include pads 36, 38 onto which the multilayer capacitor assembly 30 may be placed. Lead frames 20, 22 are soldered to the pads 36, 38 forming solder fillets 40, 42 thereby electrically connecting the stacked multilayer capacitor assembly 30 to the other electrical components on the circuit board 32.
As is known, the lead frames 20, 22 are metal and are soldered between the circuit board 32 and the multilayer capacitors 10 to provide mechanical compliance between the multilayer capacitors 10 and the circuit board 32. In this regard, the multilayer capacitors 10 often have a thermal expansion coefficient that is typically less than the circuit board 32. Consequently, during heating, the circuit board 32 expands to a greater degree than the multilayer capacitor 10. In the absence of compliance between the multilayer capacitor 10 and the circuit board 32, the multilayer capacitor 10 and/or one or more of the solder joints 24, 26 may crack or otherwise be destroyed or damaged.
Furthermore, the circuit board 32 may be more flexible than the multilayer capacitor 10. During flexing of the circuit board 32, significant mechanical stress may be applied to the multilayer capacitor 10 in the absence of the lead frames 20, 22. Lead frames 20, 22 thus provide compliance between the multilayer capacitor 10 and the circuit board 32 and allow the circuit board 32 to flex or expand upon heating while minimizing stress on the multilayer capacitors 10 and/or on the joints 24, 26.
Even with lead frames to provide compliance, manufacturers of capacitor assemblies and PCB assemblers experience failures with stacked capacitor assemblies. In particular, multilayer capacitors may be inadvertently separated from one or both of the lead frames during soldering of the multilayer capacitor to the PCB or during operation of the PCB.
While stacked capacitor assemblies have generally been successful, manufacturers strive to improve the stacked capacitor assemblies, particularly their durability and performance.